Device for converting motion into electrical energy



June 5, 1,956 A. E. SHAW EVAL. 2,749,392

DEVICE FOR CGNYERTING MOTION INTO ELECTRICAL ENERGY Original Filed Aug.16, 1946 2 Sheets-Sheet l IN VEN TORS der'. SKU/uf BY Robi/15C 9 )7700/7June 5, 1956 A, E, SHAW ETAL. 2,749,392

DEJVICE FOR CONVERTING MOTIO'N- INT0 ELECTRICAL ENERGY original med mg.'16, 194e 2 sheets-sneer 2 INVENToRs @bu/L 8, 56nd DEVICE FOR CONVERTINGMOTION INTO ELECTRICAL ENERGY Albert E. Shaw and Robert J. Moon,Chicago, Ill., as-

signors, by mesne assignments, to Louis Bernat, doing business asPhysics Research Associates Original application August 16, 1946, SerialNo. 690,992. Divided and this application September 22, 1952, Serial No.312,754

8 Claims. (Cl. 179-100.41)

This invention relates to a device for converting mechanical motion intoelectrical energy.

This application is a division of Serial No. 690,992, tiled August 16,1946, now Patent Number 2,640,170.

The principles governing the iniluence of a magnetic eld upon the pathof an electron have been known for some time. For example, it has beenshown that an electron traveling in a uniform magnetic lield directedperpendicularly to the direction of motion of the electron will becaused to travel in a circular path, and the electron will be forced toreturn to the initial point. Such behavior occurs in the magnetron, forexample, which is a vacuum tube in which the flow of electrons from thecathode to the plate is affected by a magnetic field. That is to say,for specific values of potential differences between the cathode andplate, a cutolC value of the magnetic iiux density for the currentthrough the tube may be established and all electrons leaving thecathode may be prevented from reaching the plate. In a magnetron theplate current may be varied or cut off by changing either theelectrostatic tield between the plate and cathode or the magnetic eld ina region about cutoff in accordance with the approximate formula:

where r=the radius of the cylindrical plate K=a constant for a givengeometry of cathode and anode Ec=potential of the plate with respect tothe cathode B=rnagnetic iiux density Between zero value of the appliedmagnetic eld and cutoff value thereof, the plate current may be variedfrom maximum to zero by variation of the magnetic tlux density.

A principal object of our invention is to utilize the above principlesof interaction between a moving electron and a magnetic iield in orderto provide a simple and effective device for controlling the current inan electric circuit in response to variations in movement of amechanically operated member. One example of the use to which a deviceso constructed may be put, is as a pickup for translating theundulations of a recorded disc of music or speech into electricalimpulses, as in a phonograph which utilizes vacuum tube amplification.

Another object of our invention is to provide a device as aforesaidwhich employs a diode, namely, an evacuated envelope enclosing a heatedcathode and an anode adjacent thereto, together with a permanent magnetand associated magnetic circuit, which magnet is disposed inpredetermined relation with the diode and a member of magnetic materialassociated with the diode and magnetic circuit, the member beingmechanically connected to the medium initiating the mechanical impulses.

A still further object is to provide a device as aforesaid in which themovable member is of high magnetic pernited States Patent ICC meabilityin order to influence the field of the magnet within the interelectrodespace to the highest possible degree.

Another object is to provide a device as aforesaid in which theundesirable factor of inertia of the moving parts is reduced to anegligible amount and thus renders the device responsive to a wide rangeof frequencies.

A still further object is to provide a device as aforesaid which iscompact and of small dimensions, thereby suiting the device toapplications wherein light weight and compactness are of substantialimportance, for example in connection with a phonograph pickup.

Another object is to provide a device as aforesaid which is simple andrugged in construction.

In carrying out our invention in one form, we provide a diode comprisingan evacuated glass envelope and enclosing a central elongated cathodeand a cylindrical er annular anode arranged thereabout, the saidelectrodes being carried on the usual combined supports and leads sealedin the wall of the envelope. For establishing a magnetic iield in theinterelectrode space, there is provided a permanent magnetic circuit ofU-form having hollow cylindrical pole7 pieces constituted by connectingthe legs of the yoke by means of a hollow cylinder and peripherallygrooving the cylinder to yield an air gap. rPhe diode is disposed withinthe interior of the cylinder and coaxially thereof and with thelongitudinal midpoint of the anode substantially in alignment with thegap whereby iiux across the gap is distributed through theinterelectrode space. Part of the flux passes across the gap and throughan annulus positioned adjacent to the gap. The annulus is supported foroscillating movement on a suitable pivot and is mechanically connectedto the source of mechanical movement. A suitable circuit for heating thecathode and for causing electron iiow between the cathode and anode orany suitable source of electron flow therebetween is provided, togetherwith a. circuit for impressing variations in plate current of thel diodeon an external circuit which may be an amplier.

In the gures, which show certain forms in which the invention may beembodied,

Fig. 1 is a perspective view of one embodiment of the invention;

Fig. 2 is a cross-sectional view of a part of the device of Fig. l;

Fig. 3 is a partial cross-sectional view comprising a part of Fig. 2,but on a larger scale;

Fig. 4 is a side elevational view of an alternative form of theinvention;

Fig. 5 isa cross-sectional view taken on the line 5 5 of Fig. 4;

Fig. 6 is a side elevational view of another alternative form of ourinvention;

Fig. 7 is a cross-sectional View taken on the line 7--7 of Fig. 6;

Fig. 8 is a somewhat diagrammatic transverse cross section through thediode to indicate the magnetic eld and flow of electrons;

Fig. 9 shows a schematic circuit for use with the device;

Fig. 9a shows another form of schematic circuit for use with the device;

Fig. l0 is a detailed view showing one manner of connecting a phonographneedle to the device;

Fig. ll is a cross-sectional View showing our invention embodied in amicrophone; and

Fig. l2 is a representation of a characteristic curve of one deviceconstructed in accordance with our invention, together with a graphicalshowing of a time-displacement input signal and a time-voltage outputsignal.

Patented June 5, 1956 Adverting first to Figs. l and 2, one preferredform of thedevice comprises a diode 10 formed by enclosing in a glassenvelope 11 a cathode 12 and anode 13. Although the cathode 12 thereshown is of the directly heated type, it will be understood that anindirectly heated cathode may be substituted. The cathode 12 issupported at its ends in a position substantially coaxial with the anode13 by support wires 14 which serve to supply current to the cathode.Prong terminals of any common type carried by a base cemented to theenvelope may be provided. If desired, a spring may be interposed betweenthe cathode proper and one of its supports in order to accommodatechanges in length thereof during heating and cooling.

Preferably the cathode comprises substantially nonmagnetic material, ofwhich an alloy of chromium and nickel (Nichrome V) is a suitable type.Moreover, the cathode spring, leads, and supports are preferablynonmagnetic, e. g., stainless steel, tantalum-tungsten alloy, tungsten,molybdenum, or tantalum. preferably of the coated type to improveelectron-emitting eiciency, e. g., such coating may comprise 60 per centbarium oxide (BaO) and 40 per cent strontium oxide (SrO).

Anode 13 is disposed adjacent to cathode 12 and is preferablycylindrical in form. The spacing between the cathode and anode willdepend upon the characteristics which the device is intended to possess.For example, the diode may have an envelope of internal diameter 0.31,and anode of mean diameter 0.25, and a cathode of diameter 0.0006.

To provide the magnetic circuit, there is a yoke 21 which includes thearms 22 and 23 of magnetic material of high permeability joined by abridge 24 of permanent magnetic material such as Alnico V, the threemembers being secured together by screws 25. Attached, as by tightinsertion in circular recesses 26 and 27 in the arms 22 and 23,respectively, is the hollow cylindrical pole member 28 having aperipheral groove 29 substantially centrally thereof. While thecross-sectional shape of the groove 29 may take various outlines, onepreferred form is triangular, as shown, the depth of the groove beingnearly equal to the thickness of the wall of the member 28.

Adjacent to the groove 29, and in close proximity to the exterior of themember 2S, is the annular element 31, preferably of thin, flat crosssection and supported for oscillation along the axis of the member 28 ona rocker arm 32 pivoted at 33. Although the pivot 33 is indicated moreor less schematically, as are certain othercomponents ofthe device, itwill be comprehended .that any suitable support which will permit dampedoscillations of the annular element 31 may be employed. inasmuch as theintended range of frequency of operation of the element 31 is from zeroto approximately 10,000 cycles per second, it is preferred to mount thearm 32 on a rubber or similarly resilient cushion, the elasticity ofsuch material permitting rapid oscillations and damping thereof.

At its lower extremity 35 the arm is provided with means fortransmitting mechanical impulses to the element 31. For example, inusing our device as a pickup in connection with the translation of theundulations of a recorded disc into audible sound, the lower portion 3Smay carry a chuck 36 and setscrew 37 for holding a needle 38 adapted torest in the groove of the disc, as is well known. To reduce the mass,and hence the undesirable inertia effects of the rapidly oscillatingelement 31 and arm 32, the needle 3S may be a permanent part of theportion 3S, attached as by welding or soldering (Fig. l0), therebyeliminating the mass of the chuck 36 and screw 37.

Positioned within the member 2S is the diode 10, as previously describedin detail, the same being positioned The cathode is with that medianplane of the anode which is perpendicular to the axis of said anodesubstantially in the gap 29.

Pole member 23 is a sleeve of high magnetic permeability such as softiron, whereas the yoke 24 is a high retentivity permanent magnet such asAlnico V. The dis tribution of the iiux adjacent the groove 29 issubstantially as indicated in Fig. 3. By reason of the air gap incidentto the groove 29, a substantial proportion of the lux will take a paththrough the element 31, these ilux lines being part of the entire bodyof ux lines linking the two portions of the member 2S. On the interiorof the member 2S the flux enters the interelectrode space, as indicatedschematically in Fig. 3.

It is not intended that the shape of groove 29 be limited to that shown,since other cross-sectional contours may prove equally effective. Nor isit necessary that the depth of the groove be less than the thickness ofthe wall of the member 28. That is to say, member 28 may comprise twoindependent pole pieces, one chamfered and the other blunt, arranged toabut end to end to define a space therebetween functioning similarly tothe groove 29, the desideratum being that an air gap be presented to theflux to constrain the flux to link with the element 31.

The several components of the device as just described are mounted on asupport, which may take any of several suitable forms, the sameconstituting no part of our invention. However, it has been found thatnon-magnetic materials should be used for all parts other than themagnetic yoke 21 and element 31, in order to avoid distortion due tostray magnetic fields. One method of mounting utilizes a cartridge unitadaptable to tone-arms of record players of various types, thusproviding a device readily interchangeable With other types of pickupspresently available.

To connect the device in the circuit of which it forms a part, the leads14 may be connected as shown in Fig. 9. Filament or cathode current isfurnished by a suitable source of direct current, e. g., a battery 41,while plate or anode current supplied through lead 46 is similarlyprovided, e. g., from a battery 42. A resistor 43, of the order of50,000 ohms in the plate circuit, serves as a circuit element for thegeneration of a potential variation which is impressed on the conductors44 to the input of a vacuum tube amplifier 45, the output thereof beingutilized as desired, e. g., to operate a loudspeaker. It will beappreciated that the device is capable of use with other types ofcoupling into the amplifier. That is to say, transformer or other modesof coupling are equally applicable.

Operation of the device is as follows: The magnet circuit creates amagnetic eld through the diode 10, and particularly in theintereleetrode space. Flux forming said field is representedschematically by small crosses in Fig. 8. Assume that the cathode isemitting electrons. These electrons will leave the cathode and travel tothe plate, as indicated by the curved arrows in Fig. 8, these arrowsrepresenting typical paths of the electrons which are approximately arcsof circles of equal radii. The centers for these arcs fall upon anothercircle of the same radius, the center of the latter circle beingcoincident 'with the axis of the cathode. it will be understood fromFig. 8 that an ideal condition is there schematically represented.Assuming that the magnetic ilux density is of a value less than cutoff,point A (Fig. l2), the space current will be a maximum.

Upon displacement of the element 31 by actuation of the needle 3S orother source of mechanical movement, the shifts of position of theelement with respect to the groove 29 will distort the ilux adjacent tothe groove, including that portion thereof in the interelectrode space.

The oscillations of the element 31 superimpose on this steady state ofthe magnetic ux, which distorts the interelectrode field to cause avariation of the density of tiux lines in the interelectrode space. Suchcondition is sutiicient to provide a iield at certain zones of saidspace which is at cutoff value or lower, thereby varying the number ofelectrons reaching the plate. Accordingly the plate current is made toiiuctuate in unison with the oscillations of the element 31. Suchfluctuations in plate current are impressed across the resistor 43 andamplified and reproduced as desired.

In Figs. 4 and 5 is shown a modified embodiment of the invention, inwhich there is disposed in spaced relation to the diode and coaxiallytherewith a permanent magnet 51, preferably cylindrical in form, andhaving high retentiveness, e. g., Alnico, an alloy of aluminum, nickel,and cobalt. Suitable specifications of a magnet 51 for use with thediode 10 heretofore specified would include a length of about 0.75" anda diameter of about 0.25, the flux density being in the range of 600 to700 gauss. Preferably the end of the magnet 51 adjacent the diode isprovided with a conical recess 52 to focus the magnetic field into abundle of iiux lines substantially coextensive with the interior of theanode 13, i. e., the interelectrode space. The magnet S1 is spaced fromthe adjacent end of the diode 10 by a distance of about 0.19, andinterposed between the magnet and diode is the movable vane 53 (Fig. 5)substantially triangular in contour and about 0.02 in thickness, oneimportant consideration being that one sloping edge of the vane benormally substantially coincident with a diameter of the bundle of fiuxlines associated with the magnet 51. Moreover, the vane 53 is of metalof high magnetic permeability and is supported on one end of the rocker54 pivoted at 55. Pivot 55, chuck 56, and needle 5'7 are of the sameconstruction as described in connection with Figs. l and 2. It will beunderstood that a substantial portion of the flux leaving the pole ofthe magnet adjacent the diode is concentrated through the vane 53, dueto the high permeability thereof as compared to air.

Because of the negligible mass of the vane 53 and the mechanical partsassociated therewith, together with the damping pivot suspensiondescribed, the response of the device is instantaneous, and areproduction of the mechanical vibrations imparted to the needle 57 interms of current variations in the resistor 43 is achieved.

The use of a diode having the dimensions outlined above, a filamentcurrent of 30 milliamperes at 1.5 to 3 volts, and a plate current of l5to 20 microamperes at 20 volts, combined with a flux density of 600 to700 gauss, has produced good results. Another example utilized a fiuxdensity of 1500 gauss and was responsive to incremental changes in thedensity of as low as 50 gauss. However, such specifications and examplesare indicative only and are not intended as limitative of the invention.Important factors influencing the design of the diode are the necessityfor accurate spacing between cathode and anode and the avoidance ofmagnetic materials in the device as a whole, excepting, as mentioned,the magnetic circuit; i. e., yoke 21 or magnet 51, annulus 3l, and vane53.

In Figs. 6 and 7 is depicted an alternative form of our inventionidentical in most respects with the embodiment of Figs. 4 and 5hereinabove described, but provided with a yoke 71 of solid iron orlaminations thereof, substantially of U form as shown, and having themagnet 51 afiixed to the bight thereof. The legs of the U are bentinwardly, as indicated, to form pole pieces 72, and the dimensions ofthe yoke are such that the pole pieces 72 are disposed adjacent to thebase end of the cathode and anode, as shown. The fiux lines from themagnet 51 through the interelectrode space and to the pole pieces 72 areindicated somewhat diagrammatically at 73. By em* ploying the yoke 7l inassociation with the magnet 51, a magnetic return path is provided toconcentrate the flux in the interelectrode space to a greater degree.However, when the device is to be used as a pickup, extreme lightness ishighly important, and a yoke as described would be a disadvantage. Forthose applications wherein greater weight is a relativelyinconsequential matter, the form of Figs. 6 and 7 is to be preferred. Itwill be appar'- 6 ent that in the embodiment of Figs. 6 and 7, therocker arm 54a will be offset with respect to the vane 53a in order toclear the yoke 71 (Fig. 7). p

Referring to Fig. 12, there is shown a graph to indicate therelationship between plate current and fiux density for a value of loadresistance 43 to 50,000 ohms. The par-y ticular characteristic curvethere shown is the result of a` plate supply of l2 volts. Cutoff of thefield begins at a point indicated at A, and complete cutoff would benear zero value of plate current. We prefer to operate our device at thepoint B, which is the mean value approximately midway between thebeginning and end of cutoff, and in which zone the curve issubstantially a straight line whereby to yield linear response. Signalvariations, i. e., oscillations of amplitude d as a function of time tdue to movement of the annulus 31 or vane 53, are reflected asvariations of magnetic fiux shown schematically at C, and the resultantchanges in plate current produce a voltage variation V with time tacross resistor 43. This is shown schematically at D. Although C and Dare shown for simplicity as of sine-wave form, it will be understoodthat the actual curves will consist of audio frequencies.

It will be comprehended from the description herein that the device,although illustrated in a typical environment, namely, as a pick-up fortranslating the undulations of a recording disc into electrical energy,is not limited thereto but is capable of other uses wherein it isrequired to transform discontinuities of a surface into audible orvisible intelligence. For example, one such application would be in thefield of measuring the roughness of a iinished machined surface. Thedevice of our invention may be so arranged that a feeler of the propercontour and size could be caused to trace the surface in a predeterminedpath in order that undulations or discontinuities of the surface may beimparted to the oscillatory annulus or vane to affect the device in themanner described, and thus to cause audible or visible manifestationsindicative of the nature of the surface being traced.

It will be understood that for certain applications of the device it maybe desirable to control the anode current by the use of a thirdelectrode (Fig. 9a). This may be accomplished, for example, by insertinga grid 60 between the cathode and anode, as is well known, andimpressing the necessary control voltage Ec on the grid. That is to say,the grid 69 may be used as a spacecharge grid. Moreover, the usualscreen grid and suppressor grid may be employed for achieving specifiedcharacteristics in the operation of the thermionic tube 10. Inasmuch asthe various modes of interconnecting such grid or grids for specificcontrol purposes are well known, no detailed description thereof isdeemed necessary.

One manner of adapting our device to a microphone is indicated in Fig.11, wherein the diode 10, magnet 51, and vane 81 are positioned withrespect to each other similarly to the arrangement of Fig. 4, the diodeand magnet being supported in any suitable manner in the shell 82, thevane being oscillatory with the diaphragm 83 through the medium of therod 84. Thus sound waves impinging on the diaphragm 83 will oscillatethe vane 8.1, with results as explained heretofore.

From the foregoing it will have been noted that we have provided adevice capable of translating mechanical vibrations of a wide range offrequencies into a Varying electric current for operation of anelectro-responsive device. When used as a phonograph pickup, it has manyadvantages over devices presently available. For example, when comparedto the so-called crystal pickup, it does not require any specialprotection against moisture, the crystal being highly hygroscopic andtherefore requiring careful sealing against moisture. Our device doesnot become erratic at high temperatures-for example, over F.-as does acrystal of potassium sodium tartrate (Rochelle salt). Moreover, theperformance of our device is independent of the temperature of theambient air. Because of their fragility, crystal pickups are easilydamaged in shipment, while our device is rugged and stable inadjustment.

The device of our invention is characterized by excellent fidelity, andhas high output in proportion to its weight. A conventional magnetictype of pickup for high output is of relatively heavy weight andgenerally unadaptable for use on the tone-arm of a disc type ofrcproducer.

By employing a permanent magnet of high rctentiveness, the flux in theinterelectrode space is of high density. Thus a slight change in suchlinx, due to even infinitesimal movements of the uxavarying ring orvane, will produce large changes in plate current.

Of great importance is the ability of our device to respond tofrequencies in the low range, a characteristic wherein presentlyavailable corresponding devices are inadequate. Moreover, in our deviceoutput is independent of the frequency.

While we have shown particular embodiments of our invention, it will beunderstood, of course, that we do not wish to be limited thereto sincemany modifications muy be made, and we therefore contemplate by theappended claims to cover any such modifications as fall within the truespirit and scope of our invention.

Having thus described our invention, what we claim and desire to secureby Letters Patent is:

l. A device for converting mechanical motion into electrical energycomprising a magnet including a substantially U-shaped yoke and a hollowcylindrical pole piece attached to the legs of said yoke andintermediate thereof, said piece having a peripheral groovesubstantially midway of its length, a diode supported within said pieceand positioned with the central transverse plane of the cathode andanode substantially in alignment with said recess, means for causingelectron fiow between said cathode and anode, a magnetically permeableannular member surrounding said recess, means interconnecting saidmember with the source of motion whereby said motion will actuate saidmember axially of said piece, an electrical circuit for supplying inputcurrent to said diode, and an electrically responsive circuit forreceiving the output of said diode.

2. A device for converting variations of a non-planar surface intoelectrical energy comprising a feeler adapted for relative movement withrespect to the surface to trace the variations in a predetermined order,a magnetically permeable member movable with the feeler, a thermionictube including a cathodic electrode and associated anodic electrode,means for establishing a magnetic field in the interelectrode space,said member being positioned substantially transversely to the directionof the field and within the same whereby movement of the feeler andcorresponding movement of the member influences the field in said spaceto vary electron current between the electrodes, and an electricalcircuit including said electrodes, and means for utilizing saidvariations in electron current to produce audible or visibleintelligence indicative of the variations of the non-planar surface.

3. A device for translating movement of a mechanically-actuated elementinto an audible or visible form comprising in combination a magneticallypermeable member operatively connected to the element, a thermionic tubeincluding a cathodic electrode and an anodic electrode, means forcausing electronic current fiow between said electrodes, means formaintaining a magnetic field in the interelectrode space, said memberpositioned within said magnetic field whereby movement of the membervaries the magnetic field in the interelectrode space to provide amagnetic field at certain zones of said space having a value of cutoffor lower to vary said electronic current, and an electrical circuitconnected to said tube and having components responsive to saidvariations of electronic current to provide audible or visible signalsrepresenting movement of the element.

4. A device for converting mechanically-produccd motion into electricalimpulses comprising in combination a member oscillatable by the motionand including a vane, a thermionic tube including a cathodic electrodeand an anodic electrode, means for causing electron flow from said firstto said second electrode, means for maintaining a magnetic field in theinterelectrode space, said vane being disposed to intercept at leastpart of the flux lines entering said space whereby oscillations of thevane are effective to vary the field in said space to vary the anodecurrent, and electro-responsive means connected to said electrodes fortranslating said variations of current into audible or visibleintelligence.

5. A device for converting mechanically-produced motion into electricalimpulses comprising in combination a member oscillatable by the motionand including a vane, a thermionic tube including a cathodic electrodeand an anodic electrode, means for causing electron flow from said firstto said second electrode, means for maintaining a magnetic field in theinterelectrode space, said fieldmaintaining means including a magnethaving a pole piece disposed coaxially with the axis of the cathodicelectrode and formed to focus the magnetic field into said space, saidvane being disposed to intercept at least part of the flux linesentering said space whereby oscillations of the vane are effective tovary the field in said space to vary the anode current, andelectro-responsive means connected to said electrodes for translatingsaid variations of current into audible or visible intelligence.

6. A device for converting mechanically-produced motion into electricalimpulses comprising in combination a member responsive to the motion andincluding a thin, fiat element of magnetically permeable material, themotion being in the direction of the plane thereof, a thermionic vacuumtube including a cylindrical anode and a cathode coaxially therewith,means for causing electronic current flow from said cathode to anode, amagnet including a pole piece having a conical depression in one endthereof for focussing the fiux therefrom, the pole piece being sodisposed with respect to said anode as to establish a magnetic field inthe interelectrode space, said element being disposed with its planesubstantially perpendicularly to the direction of said field and withone edge of the element substantially in alignment with a diameter ofthe focussed fiux, whereby motion of said element influences theintensity of the field in the interelectrode space to vary said currentflow in accordance with the mechanical motion, and an electrical circuitincluding said tube for converting said variations into audible orvisible intelligence.

7. A device for translating movement of a mechanically-actuated elementinto an audible or visible form comprising in combination a magneticallypermeable member operatively connected to the element, a thermionic tubeincluding a cathodic electrode, an anodic electrode, and at least onecontrol electrode associated with said first two mentioned electrodes,means for causing electronic current ow between said electrodes, meansfor impressing control voltage on said control electrode, means formaintaining a magnetic field in the interelectrode space, said memberpositioned within said magnetic field whereby movement of the membervaries the magnetic field in the interelectrode space to provide a magnetic field at certain zones of said space having a value of cutoff orlower to vary said electronic current, and an electrical circuitconnected to said tube and having components responsive to saidvariations of electronic current to provide audible or visible signalsrepresenting movement of the element.

8. A device for translating movement of a mechanically-actuated elementinto an audible or visible form comprising in combination a magneticallypermeable member operatively connected to the element, a thermionic tubeincluding a cathode and an anode, means for causing electronic currentfiow between said electrodes, means for maintaining a magnetic eld inthe interelectrode space, the ux density of said eld being of a valuesubstantially midway between zero value of the field and cutoff valuethereof, said member positioned within said magnetic field wherebymovement of the member varies the magentic eld in the region of cutol tovary said electronic current in linear relation therewith, and anelectrical cir- 10 cuit connected to said tube and having componentsresponsive to said variations of electronic current to provide audibleor visible signals representing movement of the element.

No references cited.

